1. Field of the Invention
The present invention relates to a stencil printing machine capable of conducting clear printing, and a printing drum used in the printing machine.
2. Description of the Related Art
FIG. 16 and FIG. 17 are sectional views showing a printing section of a conventional stencil printing machine. FIG. 18 is a part of a circumferential wall of a printing drum; an essential portion of said printing section. A printing drum 100 has a circumferential wall 101. The printing drum 100 is rotatably supported by an axis 102 and driven to rotate around the axis by non-illustrated driving means. In the circumferential wall 101, there is formed an opening portion 104 (printing area) having many through holes 103 formed therein and a non-opening portion 105 (non-printing area) formed around said opening portion 104. Clamping means 106 is disposed on the non-opening portion 105 in an outer circumferential surface of the circumferential wall 101. A perforated stencil sheet 107 is clamped with the leading end thereof by the clamping means 106 and wrapped around the outer circumferential surface of the circumferential wall 101. Inside the printing drum 100, an ink supplying roller 108 is situated for supplying ink to an inner circumferential surface of the circumferential wall 101. Below the printing drum 100, a press roller 109 is situated to be vertically movably relative to the outer circumferential surface of the circumferential wall 101. At least the surface of the press roller 109 is composed of an elastic material such as rubber and so on. A printing sheet is supplied in synchronization with rotation of the printing drum 100. The printing sheet is pressed against a stencil sheet 107 on the circumferential surface of the printing drum 100 by the press roller 109 moving upward. Ink is supplied to the inner circumferential surface of the printing drum 100 and passes through the opening portion 104 and a perforated portion (perforations) of the stencil sheet 107 to transfer to the printing sheet, thereby forming an image thereon.
During printing in the aforementioned constitution, as shown in FIG. 16, the press roller 109 presses the stencil sheet 107 via a not-shown printing sheet. At the same time, on the opposite side of the press roller 109 relative to the printing drum 100, the ink supplying roller 108 contacts the inner circumferential surface of the printing drum 100. In this way, during printing wherein the printing drum 100, the press roller 109 and the ink supplying roller 108 are superimposed on one another, pressure required for printing is conventionally applied only to a center portion of the printing drum 100 in the axial direction of the drum. Therefore, printing pressures exerted on both end-portions of the printing drum may be not enough and images formed there may be indistinct.
This seems to be due to insufficient ink transferring onto the printing sheet at both end-portions of the printing drum in the axial direction. This is caused by the fact that rigidity of the circumferential wall 101 of the printing drum 100 varies widely from the central opening portion 104 to the non-opening portions on both sides in the axial direction of the printing drum 100, and that pressing force against both end-portions of the printing drum by the press roller 109 is reduced since the printing drum 100 is deformed while being pressed by the press roller 109.
If the pressing force exerted on the printing drum 100 by the press roller 109 is increased to improve such problems, enough printing pressure is assured on both the end portions of the opening portion 104 in the axial direction of the printing drum 100, thereby avoiding indistinct images to be formed there. However, printing pressure on the center of the opening portion 104 consequently increases, and ink transferring onto the center of printing sheet excessively increases. This brings another problems such as seeping-through, enlargement of printed images and so on.